I think pride plays its part here. People want to prove that they could indeed cope without the person. I think there is also the point where people think, "we are going to have to learn this stuff one day, it may as well be now".

In some key-person areas long notice periods (3months, for instance) are the norm. However this is never going to be the most productive 3 months of anyone's life! Serving 3 months notice becomes very dull by the end...